1. Field of the Invention
The present invention relates to a cathode of a lithium-sulfur secondary battery, wherein the cathode is formed using a material having a relatively high content of sulfur, and methods for fabrication thereof. More particularly, the present invention provides a cathode formed from sulfur-infiltrated mesoporous conductive nanocomposites, which provides the lithium-sulfur secondary battery with a high energy density.
2. Description of the Related Art
Recently, chargeable/dischargeable secondary batteries have been widely used as large-capacity power storing batteries, such as those used in an electric vehicles and power saving systems, and as small-sized high-performance energy sources for portable electronic devices, such as a mobile phone, a camcorder, and a laptop computer.
Lithium ion batteries as secondary batteries are beneficial in that they have a relatively high energy and a relatively large capacity per unit area compared to nickel-manganese batteries and nickel-cadmium batteries.
Also, since lithium ion batteries have a low magnetic discharge rate, a long life span and no storing effect, they are convenient to use and provide a long life-span.
However, the potential for using lithium ion batteries as batteries for next generation electric vehicles is limited because they have stability issues when overheated, a low energy density, and low output.
In order to solve the problems of the lithium ion batteries, there have been many studies directed towards developing lithium-sulfur secondary batteries or lithium-air secondary batteries that may realize a high output and a high energy density.
Lithium-sulfur secondary batteries use sulfur as a cathode active material, use lithium as an anode and provide 2500 Wh/kg, i.e., 5 times a theoretical energy density of existing lithium ion batteries. Thus, these lithium-sulfur secondary batteries are suitable for use as batteries for electric vehicles that require a high output and a high energy density.
Furthermore, an abundance of sulfur for use as the cathode active material in the lithium-sulfur secondary battery exists in the earth. As such, sulfur can be provided at a low price and is expected to provide good price stability.
However, the lifespan of lithium-sulfur secondary batteries can be reduced due to the effect of self-discharge that occurs due to a polysulfide shuttle.
FIG. 1 illustrates a mechanism in which a lithium-sulfur secondary battery is charged and discharged. Theoretically, when the lithium-sulfur secondary battery is discharged, electrons that are moved from a lithium anode (Li metal) are bound to sulfur particles that are adjacent on the surface of a conductive material. The sulfur particles are thus reduced to S82− and are dissolved in an electrolyte.
Subsequently, S82− constitutes long-chain polysulfide (Li2S8) that is bound to lithium ions and is dissolved in the electrolyte. Li2S8 is finally deposited in the form of short-chain polysulfide (Li2S2/Li2S) on the surface of the lithium anode due to a continuous reduction reaction with the lithium ions.
When the lithium-sulfur secondary battery is charged, an oxidization reaction occurs, and Li2S8 is reduced to S82− after undergoing a reverse process. Thus, S82− loses electrons from the surface of the conductive material and is deposited as sulfur particles.
However, as illustrated in FIG. 1, a polysulfide shuttle phenomenon, in which Li2S8 reacts with the lithium ions during an oxidization reaction process from Li2S2/Li2S to Li2S8 and is reduced to Li2S2/Li2S, occurs when the lithium-sulfur secondary battery is charged.
In the polysulfide shuttle phenomenon, a driving force is generated by a concentration gradient of polysulfide, which prevents a problem caused by an overvoltage of the lithium-sulfur secondary battery.
However, since self-discharge occurs continuously even when the lithium-sulfur secondary battery is charged, a problem relating to a reduction in the life-span of the lithium-sulfur secondary battery occurs. Thus, when the lithium-sulfur secondary battery is charged, the efficiency of a mass of an active material is lowered.
Thus, studies have been conducted in attempts to solve the problems caused by the polysulfide shuttle phenomenon in the development of lithium-sulfur secondary batteries. For example, studies have been conducted for improving discharge capacity and life-span characteristics of lithium-sulfur secondary batteries by infiltrating sulfur into pores of mesoporous conductive materials.
FIG. 2 illustrates a technique for using a mesoporous carbon in attempt to solve the problems caused by the polysulfide shuttle phenomenon of the lithium-sulfur secondary battery. As illustrated, sulfur-infiltrated mesoporous carbon nanocomposites are synthesized by infiltrating sulfur into micropores formed in a mesoporous carbon, and a charge/discharge mechanism thereof is demonstrated.
This mechanism shown in FIG. 2 is described in U.S. Patent Publication No. 2011-0052998. In particular, as described a mesoporous carbon having micropores is first synthesized and is etched using potassium hydroxide (KOH). Mesopores are formed in inner walls of the mesoporous carbon by performing the etching process. Thereafter, a solution in which carbon disulfide is dissolved, is mixed with the mesoporous carbon. Thermal treatment is then performed on the mixture at a nitrogen atmosphere of 140° C. so as to infiltrate sulfur into the mesopores.
When charge/discharge is performed using an electrode fabricated using the described method, sulfur infiltrated into the mesopores causes a reduction reaction by receiving electrons, and the sulfur is dissolved in the state of polysulfide Sx2−.
The dissolved polysulfide is not diffused into an electrolyte, but is instead confined within the micropores and reacts with the lithium ions.
However, one problem of the described technique is a limitation in the quantity of infiltrated sulfur due to limitations on pore size and a distribution chart of the mesoporous conductive material (mesoporous carbon). As a result, it is difficult to implement a theoretical energy density with this method.
In other words, the quantity of sulfur that may be filtrated into the mesopores by the method illustrated in FIG. 2 is limited. Thus, the increase in energy density is very restricted by such use of sulfur in a battery for an electric vehicle.
Further, according to the described method, a highly-dispersed slurry should be fabricated using a nanoscale sulfur-mesoporous conductive material in order to fabricate an uniform electrode of a secondary battery. However, when the nanoscale sulfur-mesoporous conductive material is fabricated, a material yield and a supply and demand quantity thereof are very restrictive. Thus, it is difficult to use the mesoporous conductive material as an electrode of a battery for an electric vehicle.
Still further, the size of the mesoporous conductive material should be very small in order to provide micropores and mesopores. However, and when a nanoscale carbon material is fabricated, the material yield is low and thus, it is difficult to perform mass production.